1. Field of the Invention
The present invention relates to a video library system for reading a plurality of video data simultaneously from data storage devices such as disk devices in which many video data including audio data are recorded by being compression coded, which is utilized in the video-on-demand system for providing a plurality of video data simultaneously through a network in response to requests from a plurality of user terminals.
2. Description of the Background Art
As a conventional multiplexed video data reading device for reading a plurality of video data simultaneously, there has been a proposition of a video library system as disclosed in Japanese Patent Application Laid Open No. 4-269087 (1992). FIG. 1 shows a configuration of such a conventional video library system, in which a disk device DK101 is connected with a plurality of buffer memories BM101, BM102, . . . , BM132 through a switch SW101, where all of these elements are operated under a control of a control device CN101. Here, the buffer memories BM101, BM102, . . . , BM132 are provided within terminals TE101, TE102, . . . , TE132 which also include decoders DE101, DE102, . . . , DE132, and monitors TV101, TV102, . . . , TV132, respectively.
In this conventional video library system of FIG. 1, the multiplexed reading operation is controlled according to the timing chart of FIG. 2 as follows.
The control device CN101 generates a control time-slot sequence CTS101 formed by periodic 32 time-slots TS101, TS102, . . . , TS132 constituting one period, and within each period, different video data are read out from the disk device DK101 at different time-slots in such a manner that the video data for the buffer memory BM101 of the first terminal TE101 is read out at the time-slot TS101, the video data for the buffer memory BM102 of the second terminal TE102 is read out at the time-slot TS102, and so on. Accordingly, the transfer target of the video data read out from the disk device DK101 is sequentially switched by the switch SW101 among the buffer memories BM101, BM102, . . . , BM132. The video data received at the buffer memories BM101, BM102, . . . , BM132 are then decoded into video signals by the respective decoders DE101, DE102, . . . , DE132 and displayed by the respective monitors TV101, TV102, . . . , TV132, so as to realize the multiplexed video display at a plurality of terminals TE101, TE102, . . . , TE132.
As shown in FIG. 2, in this conventional video library system, in response to the video data reading request from each terminal, an idle time-slot not occupied by the other terminals is searched out in each period of the control time-slot sequence, and thereafter the reading of the requested video data is carried out periodically by using the searched out time-slot in each period of the control time-slot sequence. Consequently, there has been a waiting period for the video data reading operation between a time at which the video data reading request has arrived and a time at which the idle time-slot is found in the control time-slot sequence. Here, the response time required from a time at which the video data reading request is made to a time at which the reading of the requested video data from the disk device DK101 is actually completed for the first time-slot part becomes worst when there is only one idle time-slot in one period of the control time-slot sequence and the video data reading request arrives immediately after that idle time-slot, in which case the waiting period is as long as the duration of one period in the control time-slot sequence.
For example, in FIG. 2, consider a case in which the n-th terminal issued the video data reading request REQ101 at a timing of the completion of the time-slot TS132. First, the idle time-slot is searched among the following time-slots for one period starting from that timing. Here, suppose that the time-slots TS101 to TS131 are already occupied and only the time-slot TS132 is idle. Then, The video data reading request REQ101 is going to be allocated to this time-slot TS132, so that the execution of the video data reading operation must wait until this time-slot TS132 in the next period. In this case, the monitor display start timing RES101 at the n-th terminal is delayed from the timing of the video data reading request REQ101 by exactly one period part as indicated in FIG. 2 as TIME101. When a number of time-slots in one period is 32 and a duration of each time-slot is 125 ms, the duration of this delay time TIME101 amounts to 4 sec.
In such a conventional multiplexed video data reading scheme, in order to shorten the worst response time, it has been necessary to shorten the duration of one period by shortening the duration of each time-slot.
On the other hand, at the boundary of the adjacent time-slots in the time division multiplexed data reading control scheme, in general, there is an overhead time required for switching the data to be read out at the data storage device side. For example, in a case of using the magnetic disk device as the data storage device, there is a need to switch the reading target block on the disk, and there is an overhead time due to the head seek and the sector search for this purpose. Consequently, when the duration of each time-slot is shortened, a rate of the actual data reading time within each time-slot is going to be relatively lowered, and this in turn reduces the effective average data reading speed at the data storage device.
For example, when the overhead time is constantly 50 ms, for the time-slot with a duration of 125 ms, the reading efficiency at the disk device is going to be 60% (=(125-50)/125), but when the duration of the time-slot is shortened to 62.5 ms, the reading efficiency at the disk device is going to be lowered to 20% (=(62.5-50)/62.5), so that the average data reading speed is going to be reduced to 1/3.
Thus, in the conventional multiplexed video data reading scheme, there has been a problem that, when each time-slot is shortened in order to shorten the worst response time, the average data reading speed at the data storage device is lowered in turn, and as a result the possible level of multiplexing is going to be lowered.
In addition, in the conventional video library system, each terminal is given no information concerning the video data contents, so that in order to distinguish the video data having similar attributes such as the video data having similar titles, it has been necessary for each terminal to carry out an operation to actually make an access to a center to read out these video data and confirm their contents.